Declare support for Python 3.9 (#138)

Author: hugovk

setup.py

@@ -60,7 +60,8 @@
         'Programming Language :: Python :: 3.5',
         'Programming Language :: Python :: 3.6',
         'Programming Language :: Python :: 3.7',
-        'Programming Language :: Python :: 3.8'
+        'Programming Language :: Python :: 3.8',
+        'Programming Language :: Python :: 3.9',
     ],
     project_urls={
         'Documentation': 'https://certifiio.readthedocs.io/en/latest/',